An actuator for electromagnetically controlling a valve consists essentially of an opening magnet and a closing magnet separated from one another by a component made of non-ferromagnetic material and designed, for example, as a housing part. The opening magnet and the closing magnet are electromagnets, each comprising an operating coil and a yoke. Between opening magnet and closing magnet there is a retaining plate made of ferromagnetic material and moved in the respective direction by energizing the operating coil of the opening magnet or the operating coil of the closing magnet. The opening magnet has a bushing for a plunger which transmits the forces acting on the retaining plate to at least one gas change valve. In some actuator designs, the closing magnet also has a bushing in which a pushrod is located that transmits the forces acting on the retaining plate via an actuator spring plate to an actuator spring placed in a formed shape of the closing magnet.
In the case of actuators without pushrod, the actuator spring is as a rule placed between an actuator spring plate located on the plunger and the outside of the opening magnet.
The plunger and, where applicable, the pushrod, are mounted in guide sleeves that are built into the bushing in the yoke of the opening magnet and in the yoke of the closing magnet. The guide sleeves have channels through which the plunger oscillating in the guide sleeve and, where applicable, the pushrod are supplied with lubricant.
An actuator forms together with a gas change valve a functional unit, where the gas change valve, corresponding to a conventional cylinder head with camshafts, is drawn into the valve seat of the cylinder head by means of a valve spring and a valve spring plate.
If a functional unit comprising an actuator and a gas change valve is fitted to the internal combustion engine, the actuator spring and the valve spring are preloaded and at least one gas change valve, the plunger with the retaining plate and, where applicable, the pushrod are pushed against one another.
In the non-operated position of the functional unit, the retaining plate is located precisely in the center between the opening magnet and the closing magnet. The gas change valve is then in a central position between the valve seat of the cylinder head and the position in which the valve is opened to the maximum.
When starting up an actuator from the non-operated state, there is an initial transient state in which, for example, the operating coils of the two electromagnets are supplied with current alternately. As initial transient frequency of the spring-mass system, a frequency is selected that is preferably in the proximity of the resonant frequency which is due to the oscillating mass of the functional unit and to the resetting force of the valve spring and of the actuator spring.
A typical initial transient state is shown in FIG. 3. The operating coils of the electromagnets were each supplied here with current eight times before the closing magnet drew the gas change valve completely into the valve seat. Currents of up to 30 amperes were reached in the process.
When the internal combustion engine is in operation, the operating coils of the actuators are supplied with current according to precisely dimensioned current curves in order to position the gas change valves exactly. These current curves are usually controlled in a closed loop by, for example, determining the actual position of an oscillating component through a sensor arrangement of the control loop, and correcting the current curves of the operating coils accordingly when a deviation occurs between the desired position and the actual position of the oscillating component.
One disturbance leading to the occurrence of large deviations is the temperature-dependent frictional resistance of the actuator. Particularly when starting the internal combustion engine from cold at low outside temperatures, the increased resistance of the lubricant due to it still being cold and viscous produces the problem of the operating coils of the actuators being subjected to current curves for the initial transient state having very high current values. The current source made available for the actuators must therefore be designed to provide very high currents. Furthermore, an increased number of alternating energizing cycles is required for the operating coils. High deviations result in an increased number of control cycles until the specific actual values in the control loop correspond to the preset desired values.